Dip filter frequency characteristic decision method

ABSTRACT

Resonant frequencies f 2  and f 3  detected in a resonant space are determined as center frequencies of a dip. Based on measurement values at a speaker and a microphone in the resonant space, a basic amplitude frequency characteristic Ca and a target amplitude frequency characteristic Cd are found. A smoothness degree on a frequency axis is larger in the target amplitude frequency characteristic Cd than the basic amplitude frequency characteristic Ca. A damping level and quality factor of the dip are determined based on a difference between the basic amplitude frequency characteristic Ca and the target amplitude frequency characteristic Cd in the center frequencies f 2  and f 3  of the dip and frequencies near the center frequencies.

The present application claims the benefit of priority of InternationalPatent Application No. PCT/JP2004/002141 filed on Feb. 24, 2004, whichapplication claims priority of Japanese Patent Application No.2003-51147 filed Feb. 27, 2003. The entire text of the priorityapplication is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method of determining a frequencycharacteristic of a dip filter used for preventing resonance in a spacein which acoustic equipment is installed.

BACKGROUND ART

For example, when acoustic equipment such as a speaker is installed in ahall or a gymnasium to radiate a loud sound wave from the speaker, musicor voice from the speaker is sometimes difficult to listen to because ofthe presence of a resonant frequency in this space (loud sound space inwhich the acoustic equipment is installed). To be specific, if the loudsound wave from the speaker contains a component of the resonantfrequency in large amount, resonance occurs in a frequency of thiscomponent in the loud sound space. A resonant sound is like “won . . . ”or “fan . . . . ” The resonant sound is not a sound wave to be radiatedfrom the speaker and makes it difficult to listen to the music or thevoice from the speaker.

To avoid this, the resonant frequency in the loud sound space isdetected, and a dip filter or the like is provided at a forward stage ofthe speaker in the acoustic equipment to remove the component of theresonant frequency. Thereby, resonance is unlikely to occur in this loudsound space, making it easy to listen to the music or the voice from thespeaker.

In order to enable the dip filter to give such effects, it is necessaryto determine a frequency characteristic of the dip filter so that theresonant frequency in this loud sound space is a frequency to beremoved.

Traditionally, an operator or a measuring person for the acousticequipment distinguished the loud sound from the speaker or the resonantsound depending on their senses of hearing to make judgment of theresonant frequency, and the resonant frequency was set in the dip filteras the frequency to be removed. And, a damping level (depth) or qualityfactor (Q) of the dip filter was set so as to prevent resonance.

Even if the operator or the measuring person can distinguish theresonant frequency, it is not easy to set the frequency characteristicof the dip filter. In particular, it is not easy to appropriately setthe damping level (depth) or the quality factor (Q) of the dip. Thedamping level of the dip is maximized (depth is maximized) and thequality factor is minimized (Q is minimized) if priority is given toonly prevention of resonance. But, if the damping level becomes too highor the quality factor becomes too low, a sound quality of the acousticequipment may be degraded, or music or voice may be difficult to listen.

Some skill or experience is required to set the damping level or thequality factor of the dip appropriately in order to avoid occurrence theabove mentioned event. These factors (damping level or the qualityfactor of the dip) are not accurately set if the setting depends on theskill or experience. Furthermore, this has impeded automatic measurementand automatic adjustment of the acoustic equipment installed in the loudsound space or the like.

DISCLOSURE OF THE INVENTION

The present invention has been developed in view of the above describedproblems, and an object of the present invention is to provide a methodof determining a frequency characteristic of a dip filter which iscapable of accurately determining a characteristic of a dip filterwithout a need for experience or skills.

In order to solve the above mentioned problems, a method of determininga frequency characteristic of a dip filter of the present inventioncomprises determining a resonant frequency detected in a resonant spaceas a center frequency of a dip; finding a basic amplitude frequencycharacteristic based on a measurement value obtained by outputting aloud sound wave of a predetermined measurement signal from a speakerplaced in the resonant space and by receiving the loud sound wave in amicrophone placed in the resonant space; finding a target amplitudefrequency characteristic having a smoothness degree on a frequency axiswhich is larger than a smoothness degree of the basic amplitudefrequency characteristic on the frequency axis, based on the measurementvalue; and determining a damping level and/or quality factor of the dipbased on a difference between the basic amplitude frequencycharacteristic and the target amplitude frequency characteristic in thecenter frequency and a frequency near the center frequency.

In accordance with this method, the amplitude frequency characteristichaving the smoothness degree on the frequency axis that is larger thanthat of the basic amplitude frequency characteristic is assumed as thetarget amplitude frequency characteristic. Therefore, the targetamplitude frequency characteristic is objectively found, and based onthis, the damping level or quality factor of the dip are objectivelydetermined.

In the above method, the target amplitude frequency characteristic maybe obtained by smoothing according to any method, for example, by movingaverage of the measured amplitude frequency characteristic on thefrequency axis.

The method may further comprise determining a damping level and/orquality factor of the dip so that a second area is substantially equalto a first area; wherein the first area is an area defined by a curve ofthe basic amplitude frequency characteristic and a curve of the targetamplitude frequency characteristic in a frequency range from a firstfrequency to a second frequency when the curve of the basic amplitudefrequency characteristic and the curve of the target amplitude frequencycharacteristic are represented in an amplitude frequency characteristicview in which a logarithm axis indicating an amplitude level is anordinate axis and an axis indicating a frequency is an abscissa axis;wherein the first frequency is closest to the center frequency of thedip among frequencies at which the curve of the basic amplitudefrequency characteristic and the curve of the target amplitude frequencycharacteristic cross each other, the frequencies being lower than thecenter frequency of the dip; wherein the second frequency is closest tothe center frequency of the dip among frequencies at which the curve ofthe basic amplitude frequency characteristic and the curve of the targetamplitude frequency characteristic cross each other, the frequenciesbeing higher than the center frequency of the dip; and wherein thesecond area is an area of the dip formed when a characteristic of thedip is represented on the amplitude frequency characteristic view inwhich the logarithm axis indicating the amplitude level is the ordinateaxis and the axis indicating the frequency is the abscissa axis.

In accordance with the above method, since the area formed by exceedingthe basic amplitude frequency characteristic curve from the targetamplitude frequency characteristic curve is substantially equal to thearea of the dip, a characteristic near the target amplitude frequencycharacteristic is achieved by applying the characteristic of the dip tothe basic amplitude frequency characteristic.

The method may further comprise determining the damping level of the dipso that the damping level substantially conforms to an amplitude leveldifference in the center frequency of the dip between the basicamplitude frequency characteristic and the target amplitude frequencycharacteristic; and determining the quality factor of the dip so thatthe second area is substantially equal to the first area.

By applying the characteristic of the dip determined by such a method tothe basic amplitude frequency characteristic, a characteristic which isextremely near the target amplitude frequency characteristic isachieved.

The method may further comprise determining the center frequency of thedip in such a manner that a resonant frequency with the highestamplitude level of the second amplitude frequency characteristic is setas the center frequency of the dip among plural resonant frequencies andremaining resonant frequencies are not set as the center frequency ofthe dip when the plural resonant frequencies detected in the resonantspace are included in the frequency range from the first frequency tothe second frequency; wherein the second amplitude frequencycharacteristic is obtained by outputting, from the speaker, a loud soundwave of a synthesized signal containing the measurement signal and asignal output from the microphone and by receiving the loud sound wavein the microphone.

In accordance with this method, it is possible to avoid setting ofunwanted dips in the dip filter.

The method may further comprises detecting the resonant frequency of theresonant space based on comparison between a first amplitude frequencycharacteristic and a second amplitude frequency characteristic; whereinthe first amplitude frequency characteristic is obtained based on themeasurement value; and wherein the second amplitude frequencycharacteristic is obtained by outputting, from the speaker, a loud soundwave of a synthesized signal containing the measurement signal and asignal output from the microphone and by receiving the loud sound wavein the microphone.

The second amplitude frequency characteristic according to this methodis an amplitude frequency characteristic in a system including afeedback loop in which a signal output from a microphone is input to aspeaker. This feedback loop causes the second amplitude frequencycharacteristic to show a noticeable effect of the resonance of theresonant space in contrast to the first amplitude frequencycharacteristic. Therefore, by comparing between the first amplitudefrequency characteristic and the second amplitude frequencycharacteristic, the resonant frequency in the resonant space can beaccurately detected.

In the method, a peak frequency at which an amplitude of the secondamplitude frequency characteristic is larger than an amplitude of thefirst amplitude frequency characteristic may be detected as the resonantfrequency of the resonant space, from a difference between the firstamplitude frequency characteristic and the second amplitude frequencycharacteristic.

In the method, the measurement signal may be effective as a sine wavesweep signal.

These objects as well as other objects, features and advantages of theinvention will become more apparent to those skilled in the art from thefollowing description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a construction of an acoustic systeminstalled in a loud sound space;

FIG. 2 is a schematic block diagram of a system for measuring anamplitude frequency characteristic of the loud sound space;

FIG. 3 is a schematic block diagram of a system for measuring anamplitude frequency characteristic of the loud sound space;

FIG. 4 is a view schematically showing a first amplitude frequencycharacteristic of the loud sound space which is measured by the systemof FIG. 2 and a second amplitude frequency characteristic of the loudsound space which is measured by the system of FIG. 3;

FIG. 5 is a view of a frequency characteristic showing an amplitudelevel difference between the first frequency characteristic of a curveCa in FIG. 4 and the second frequency characteristic of a curve Cb inFIG. 4;

FIG. 6 is a view of a frequency characteristic obtained by extractingthe curve Cb from the frequency characteristic in FIG. 4;

FIG. 7 is a view of a frequency characteristic showing a curve Ca of abasic amplitude frequency characteristic and a curve Cd of a targetamplitude frequency characteristic;

FIG. 8 is a view of a frequency characteristic showing three candidatefrequencies in a frequency range from a frequency f61 to a frequencyf62; and

FIG. 9 is a view showing an amplitude frequency characteristic of a dipin which a center frequency is a frequency f2.

BEST MODE FOR CARRYING OUT THE INVENTION

A method of determining a frequency characteristic of a dip filteraccording to an embodiment of the present invention will be describedwith reference to the drawings.

FIG. 1 is a schematic view of a construction of an acoustic systeminstalled in a loud sound space (e.g., concert hall or gymnasium) 40.The acoustic system comprises a sound source device 2, a dip filter 4,an amplifier 12, and a speaker 13. The sound source device 2 may be amusic instrument such as a CD player for playback of, for example, musicCD, or a microphone. While the sound source device 2 is illustrated asbeing located outside the loud sound space 40 in FIG. 1, it mayalternatively be located within the loud sound space 40. The soundsource device 2 may be, for example, a microphone installed within theloud sound space 40. The dip filter 4 serves to remove a signalcomponent in a specified frequency from a signal output from the soundsource device 2 and to output the resulting signal to the amplifier 12.The amplifier 12 amplifies the signal output from the dip filter 4 andoutputs the amplified signal to the speaker 13, which outputs a loudsound wave in the loud sound space 40.

When the loud sound space 40 has a resonant frequency and the loud soundwave output from the speaker 13 contains a component of the resonantfrequency in large amount, resonance occurs in the loud sound space 40and thereby music or voice output from the speaker 13 is difficult tolisten to. If an appropriate frequency characteristic is set in the dipfilter 4 in this acoustic system, then the resonance in the loud soundspace 40 is prevented without degrading a sound quality of the loudsound wave from the speaker 13.

In this embodiment, a frequency to be set in the dip filter 4 isdetermined. First of all, a method and device for detecting the resonantfrequency in the resonant space 40 will be described with reference toFIGS. 2 to 5.

FIG. 2 is a schematic block diagram of a system A for measuring anamplitude frequency characteristic of the loud sound space (e.g.,concert hall or gymnasium) 40. The system A comprises a transmitter 11which is a sound source means configured to output a measurement signal,an amplifier 12 configured to receive, as an input, the signal outputfrom the transmitter 11 and to power-amplify the signal, a speaker 13configured to receive, as an input, the signal output from the amplifier12 and to output a loud sound wave, a microphone 14 configured toreceive the loud sound wave radiated from the speaker 13, and a meter 15configured to receive, as an input, the sound wave from the microphone14. The microphone 14 may be a noise meter.

The speaker 13 and the microphone 14 are placed within the loud soundspace 40. The microphone 14 is positioned to be sufficiently distantfrom the speaker 13 within the loud sound space 40. The microphone 14 ispositioned so as to receive a reflected sound of the sound wave directlyoutput from the speaker 13 at a sufficiently high level within the loudsound space 40.

The transmitter 11 outputs, as the measurement signal, a sine wavesignal whose frequency varies with time, i.e., a sine wave sweep signal.The sine wave sweep signal has a constant sine wave level at respectivetime points during frequency sweep.

The meter 15 has a band pass filter whose center frequency varies withtime. The band pass filter varies the center frequency with timeaccording to time variation of the frequency of the sine wave sweepsignal output from the transmitter 11. Therefore, the meter 15 detectsthe level of the signal which has been received as an input from themicrophone 14 and has passed through the band pass filter, thusmeasuring an amplitude characteristic of the frequency at that point oftime.

FIG. 3 is a schematic block diagram of a system B for measuring anamplitude frequency characteristic of the loud sound space 40. Thesystem B is constructed such that a signal synthesization path is addedto the system A of FIG. 2. To be specific, the system B of FIG. 3comprises the transmitter 11 which is the sound source means configuredto output the measurement signal, a mixer 16, the amplifier 12configured to receive, as an input, a signal output from the mixer 16and to power-amplify the signal, the speaker 13 configured to receive,as an input, the signal output from the amplifier 12 and to output aloud sound wave, the microphone 14 configured to receive the loud soundwave radiated from the speaker 13, and the meter 15 configured toreceive, as an input, the sound wave output from the microphone 14.

The speaker 13 and the microphone 14 are placed at the same positionswithin the loud sound space 40 as those in the system A of FIG. 2. Thetransmitter 11, the amplifier 12, the speaker 13, the microphone 14, andthe meter 15 in the system B of FIG. 3 are identical to those in thesystem A of FIG. 2.

The difference between the system B of FIG. 3 and the system A of FIG. 2is that the amplifier 12 receives, as the input, the signal output fromthe transmitter 11 in the system A of FIG. 2, while the amplifier 12receives, as the input, the signal output from the mixer 16 in thesystem B of FIG. 3. The mixer 16 of FIG. 3 receives, as inputs, themeasurement signal (sine wave sweep signal) output from the transmitter11 and the loud sound wave from the microphone 14, synthesizes (mix)these signals, and outputs a synthesized signal (mixed signal).

Thus far, the method of measuring the amplitude frequency characteristicof the loud sound space 40 in the system A of FIG. 2 and the method ofmeasuring the amplitude frequency characteristic of the loud sound wave40 in the system B of FIG. 3 have been described. Hereinbelow, theamplitude frequency characteristic of the loud sound space 40 which ismeasured by the system A of FIG. 2 is referred to as a first amplitudefrequency characteristic and the amplitude frequency characteristic ofthe loud sound space 40 which is measured by the system B of FIG. 3 isreferred to as a second amplitude frequency characteristic.

FIG. 4 is a view of a frequency characteristic schematically showing afirst amplitude frequency characteristic of the loud sound space 40which is measured by the system A of FIG. 2 and the second amplitudefrequency characteristic of the loud sound space 40 which is measured bythe system B of FIG. 3. In FIG. 4, an ordinate axis and an abscissa axisare logarithmic axes and indicate an amplitude level and a frequency,respectively. As used herein, the term “amplitude level” refers to alogarithm of a ratio of an amplitude value (magnitude of amplitude) to areference value (magnitude of reference), and is typically representedby “dB.” In FIG. 4, a curve Ca indicated by a solid line is the firstamplitude frequency characteristic measured by the system A of FIG. 2and a curve Cb indicated by a broken line is the amplitude frequencycharacteristic measured by the system B of FIG. 3.

Both the system A of FIG. 2 and the system B of FIG. 3 measure amplitudevalues at a number of frequency points. For example, in a range offrequencies to be measured, the systems A and B measure the amplitudevalues at intervals of 1/192 octave. The measurement values at a numberof points (a number of frequency points) may be indicated by the curvesCa and Cb as the first and second amplitude frequency characteristics ofthe loud sound space 40 without being smoothed on a frequency axis, orotherwise may be indicated by the curves Ca and Cb after they aresmoothed on the frequency axis in some method or another. Themeasurement values may be smoothed in various methods, including movingaverage, for example. By way of example, moving average of 9 points maybe performed for the measurement values at a number of frequency pointson the frequency axis. When the smoothed measurement values are used asthe curve Ca, the smoothed measurement values are desirably used as thecurve Cb. In this case, the curve Cb is desirably obtained by the samesmoothing method as the curve Ca. If the curve Ca is obtained byperforming moving average of 9 points on the frequency axis, then thecurve Cb is desirably obtained by performing moving average of 9 pointson the frequency axis.

The first amplitude frequency characteristic indicated by the curve Caof FIG. 4 contains the resonant characteristic of the loud sound space40 as well as the characteristic of the acoustic system including theamplifier 12, the speaker 13, and the microphone 14. The secondamplitude frequency characteristic indicated by the curve Cb of FIG. 4also includes the resonant characteristic of the loud sound space 40 aswell as the characteristic of the acoustic system including theamplifier 12, the speaker 13, and the microphone 14. The secondamplitude frequency characteristic indicated by the curve Cb shows anoticeable effect of the resonant characteristic of the loud sound space40 by a feedback loop in which the signal output from the microphone 14is input to the amplifier 12 and is output from the speaker 13, incontrast to the first amplitude frequency characteristic of the curveCa. Therefore, based on the difference between the curves (curve Ca andcurve Cb), the resonant characteristic of the loud space 40 is known.

The frequency characteristic of FIG. 5 is obtained by subtracting thecharacteristic of the curve Ca from the characteristic of the curve Cbof FIG. 4, i.e., an amplitude level difference between the firstamplitude frequency characteristic of the curve Ca and the secondamplitude frequency characteristic of the curve Cb. In FIG. 5,frequencies having positive peaks in the curve Cc are frequencies f1,f2, and f3. It is probable that among these, the frequencies havinglarger peaks are resonant frequencies of the loud sound space 40. Thepossibility that the frequency f3 having the largest peak is theresonant frequency of the loud sound space 40 is the highest. Thepossibility that the frequency f2 having the second largest peak is theresonant frequency of the loud sound space 40 is the second highest. Thenumber of resonant frequencies of the resonant space 40 is not limitedto one, but may be in many cases more. This follows that one or more ofthe frequencies f1, f2, and f3 may be the resonant frequency. Based onthe characteristic of FIG. 5, the frequencies which may be the resonantfrequencies can be objectively detected.

Thus far, the method and device for detecting the resonant frequency inthe resonant space 40 have been described with reference to FIGS. 2 to5.

Subsequently, how to determine the frequency characteristic of the dipfilter 4 of the acoustic system of FIG. 1 based on the resonantfrequencies (frequencies f1, f2, and f3) detected as described abovewill be described.

The curve Ca of FIG. 4 is the first amplitude frequency characteristicof the loud sound space 40 which is obtained by the system A of FIG. 2.The resonant frequency is detected based on the curve Ca as describedabove. The characteristic of the curve Ca is hereinafter referred to as“basic amplitude frequency characteristic.” Measurement values at anumber of frequency points by the system A of FIG. 2 may be smoothed onthe frequency axis or not to obtain the “basic amplitude frequencycharacteristic.”

First, the frequencies f1, f2, and f3 are obtained as the frequencieshaving the positive peaks from the frequency characteristic curve Cc ofFIG. 5. It is highly probable that these frequencies are the resonantfrequencies in the loud sound space 40. From them, predeterminedfrequencies are selected as candidates for the dip center frequencies tobe set in the dip filter 4 as frequencies to be removed.

Specifically, from these frequencies, candidate frequencies are selectedin decreasing order of the amplitude levels in the curve Cb of FIG. 4.

FIG. 6 is a view of a frequency characteristic obtained by extractingonly the curve Cb from FIG. 4. In FIG. 6, an ordinate axis and anabscissa axis are logarithmic axes. In FIG. 6, the ordinate axisindicates an amplitude level and an abscissa axis indicates a frequency.In the curve Cb of FIG. 6, the amplitude levels of the frequencies f1,f2, and f3 decrease in the order of f2, f3, and f1. If the number offrequencies to be selected as the candidate frequency is “three,” thenall the frequencies f1, f2, and f3 are candidate frequencies.

For example, when a number of resonant frequencies are detected, onlyfrequencies of a predetermined number may be selected as candidates ofthe center frequencies of the dip which are set in the dip filter 4 asthe frequency to be removed, rather than all the detected frequencies.For example, when a number of (200 or more) resonant frequencies aredetected, 120 frequencies may be selected as candidate frequencies, andthe remainder may be excluded from the candidate frequencies. In thiscase, the candidate frequencies may be in preference selected indecreasing order of the amplitude levels in the curve Cb of FIG. 6.

Subsequently, the candidate frequencies (frequencies f1, f2, and f3) arearranged in decreasing order of the amplitude levels in the frequencycharacteristic curve Cc in FIG. 5. The amplitude levels in the curve Ccof FIG. 5 decrease in the order of f3, f2, and f1. Therefore, at thistime, the frequency f3 is the first candidate frequency, the frequencyf2 is the second frequency, and the frequency f1 is the third frequency.

Subsequently, a target amplitude frequency characteristic is obtainedfrom the measurement values which the system A of FIG. 2 has measured ata number of points. The target amplitude frequency characteristic isobtained by smoothing the measurement values which the system A of FIG.2 has measured at a number of frequency points on the frequency axis.The smoothing method may include, for example, moving average on thefrequency axis. As described above, the measurement values measured at anumber of frequency points by the system A of FIG. 2 may be smoothed onthe frequency axis or not to obtain the curve Ca of FIG. 4 (basicamplitude frequency characteristic). But, the target amplitude frequencycharacteristic is found by smoothing such that its smoothness degree onthe frequency axis is larger than that of the basic amplitude frequencycharacteristic. If the basic amplitude frequency characteristic isobtained by moving average of, for example 9 points on the frequencyaxis, it is necessary to obtain the target amplitude frequencycharacteristic by moving average of a window width larger than 9 points(e.g., 65 points). In this manner, the target amplitude frequencycharacteristic is objectively obtained without depending on experience.

In FIG. 7, the curve Ca extracted from FIG. 4 is illustrated. In FIG. 7,an ordinate axis and an abscissa axis are logarithmic axes. In FIG. 7,the ordinate axis indicates an amplitude level and an abscissa axisindicates a frequency. The curve Ca of FIG. 7 is identical to that ofthe curve Ca of FIG. 4. In FIG. 7, a broken line Cd is illustrated. Thecurve Cd is obtained by moving average of the amplitude values measuredat a number of frequency points by the system A of FIG. 2 on thefrequency axis. Since the window width for the moving average at thistime is relatively larger, the smoothness degree of the curve Cd (targetamplitude frequency characteristic) is much larger than that of thecurve Ca (basic amplitude frequency characteristic).

The frequency f3, the frequency f2, and the frequency f1 have beenselected as the first, second, and third candidate frequencies. Then,the frequencies at which the amplitude level of the basic amplitudefrequency characteristic (curve Ca) is smaller than the amplitude levelof the target amplitude frequency characteristic (curve Cd) are excludedfrom the candidate frequencies. As can be seen from FIG. 7, in thefrequency f1, the amplitude level of the basic amplitude frequencycharacteristic (curve Ca) is smaller than the amplitude level of thetarget amplitude frequency characteristic (curve Cd). Therefore, thefrequency f1 is excluded from the candidate frequencies. As a result,the frequencies f2 and f3 are selected as the candidate frequencies.Specifically, the frequency f3 is the first candidate frequency and thefrequency f2 is the second candidate frequency.

Then, with reference to FIG. 7, frequency ranges in which the respectivecandidate frequencies are included and the curve Ca of the basicamplitude frequency characteristic continuously exceeds from the curveCd of the target amplitude frequency characteristic on a positive sideon the frequency axis are detected. As illustrated in FIG. 7, in thefrequency f3 which is the first candidate frequency, the amplitude levelis larger in the basic amplitude frequency characteristic than in thetarget amplitude frequency characteristic. In a range near the frequencyf3 on the frequency axis of FIG. 7, a frequency f31 and a frequency f32are detected as points at which the curve Ca of the basic amplitudefrequency characteristic and the curve Cd of the target frequencycharacteristic cross each other. In the frequency f31 and the frequencyf32, the curve Ca of the basic amplitude frequency characteristic andthe curve Cd of the target amplitude frequency characteristic cross eachother. The frequency f31 is closest to the frequency f3, among points atwhich the curve Ca of the basic amplitude frequency characteristic andthe curve Cd of the target amplitude frequency characteristic cross eachother in a frequency range lower than the frequency f3. The frequencyf32 is closest to the frequency f3, among points at which the curve Caof the basic amplitude frequency characteristic and the curve Cd of thetarget amplitude frequency characteristic cross each other in afrequency range higher than the frequency f3.

After detecting the frequency range (range from the frequency f31 to thefrequency f32) in which the curve Ca of the basic amplitude frequencycharacteristic continuously exceeds from the curve Cd of the targetamplitude frequency characteristic on the positive side (above) on thefrequency axis, it is detected whether or not two or more candidatefrequencies are included in this frequency range. If two or morecandidate frequencies are included, one of them is selected as thecandidate frequency and the remainder is excluded from the candidatefrequencies. The frequency to be selected as the candidate frequency isdetermined based on the magnitude of the amplitude levels in thecharacteristic in FIG. 6 (second amplitude frequency characteristic(curve Cb)). Specifically, only the frequency with the highest amplitudelevel in the second amplitude frequency characteristic (curve Cb) isselected as the candidate frequency. As shown in FIG. 7, the frequencyrange from the frequency f31 to the frequency f32 includes only thefrequency f3 as the candidate frequency, and therefore, no frequency isexcluded.

The frequency f2 which is the candidate frequency is treated in the samemanner. Specifically, a frequency range in which the frequency f2 isincluded and the basic amplitude frequency characteristic (curve Ca)continuously exceeds from the target amplitude frequency characteristic(curve Cd) on the positive side on the frequency axis is detected. Asshown in FIG. 7, in a frequency range from the frequency f21 to thefrequency f22, the basic amplitude frequency characteristic (curve Ca)exceeds continuously from the target amplitude frequency characteristic(curve Cd) on the positive side on the frequency axis. The frequency f2is included in this frequency range. Then, it is detected that whetheror not two or more candidate frequencies are included in this frequencyrange. There is no candidate frequency other than the frequency f2 inthis frequency range. So, the frequency to be excluded does not exist inthis frequency range.

Subsequently, a case where plural candidate frequencies exist in such afrequency range will be described with reference to FIG. 8. In FIG. 8, acurve Ce indicates a basic amplitude frequency characteristic and acurve Cf indicates a target amplitude frequency characteristic. Thesecurves cross each other at a frequency f61 and a frequency f62. Threecandidate frequencies (frequency f51, frequency f52 and frequency f53)exist in this frequency range (frequency range from f61 to frequencyf62).

In FIG. 8, a curve Cn indicates the second amplitude frequencycharacteristic, i.e., amplitude frequency characteristic obtained byoutputting, from the speaker 13, a loud sound wave of a synthesizedsignal containing a measurement signal (sine wave sweep signal) and asignal output from the microphone 14 and by receiving the loud soundwave in the microphone 14. Among the frequency f51, the frequency f52,and the frequency f53, the amplitude level of the second amplitudefrequency characteristic (curve Cn) of the frequency f51 is the largest.Therefore, only the frequency f51 is selected as the candidate frequencyand the remaining frequencies (frequency f52 and frequency f53) areexcluded from the candidate frequencies. This eliminates the possibilitythat the unwanted frequencies are selected as the candidate frequenciesand hence are set in the dip filter 4.

It shall be understood that if there exists a frequency with a amplitudelevel difference between the basic amplitude frequency characteristic(curve Ce) and the second amplitude frequency characteristic (curve Cn)being a predetermined level or lower (e.g., 1 dB or lower) among thecandidate frequencies (frequency f1, frequency f2 and frequency f3),this frequency is not set as the center frequency of the dip in the dipfilter 4. While the second amplitude frequency characteristic (curve Cn)of the frequency f51 has the highest amplitude level among the frequencyf51, the frequency f52, and the frequency f53, it is excluded from thecandidate frequencies and the frequency f52 having the second highestamplitude level in the second amplitude frequency characteristic (curveCn) is selected as the candidate frequency, if the amplitude leveldifference in the frequency f51 between the basic amplitude frequencycharacteristic (curve Ce) and the second amplitude frequencycharacteristic (curve Cn) is the predetermined level or lower (e.g., 1dB or lower). As a matter of course, the frequency f53 is excluded fromthe candidate frequencies.

Thus far, the case where three frequencies exist as candidatefrequencies in the frequency range has been described with reference toFIG. 8.

With reference to FIG. 7, the frequency ranges in which the candidatefrequencies are included and the basic amplitude frequencycharacteristic (curve Ca) continuously exceeds from the target amplitudefrequency characteristic (curve Cd) on the positive side on thefrequency axis have been detected. As described above, two or morecandidate frequencies do not exist in each of the detected frequencyranges, and therefore the frequency f2 and the frequency f3 are selectedas the candidate frequencies.

Next, the remaining candidate frequencies are re-arranged in order asfollows. The candidate frequencies are re-arranged in decreasing orderof amplitude level difference between the basic amplitude frequencycharacteristic (curve Ca) and the target amplitude frequencycharacteristic (curve Cd). As can be seen from FIG. 7, the amplitudelevel difference in the frequency f2 between the basic amplitudefrequency characteristic (curve Ca) and the target amplitude frequencycharacteristic (curve Cd) is 2.5 dB, and the amplitude level differencein the frequency f3 between the basic amplitude frequency characteristic(curve Ca) and the target amplitude frequency characteristic (curve Cd)is 1.8 dB. Therefore, the candidate frequencies are re-arranged in ordersuch that the frequency f2 is the first candidate frequency and thefrequency f3 is the second candidate frequency.

The frequency f2 which is the first candidate frequency is determined asthe center frequency of the dip (frequency to be removed) in the dipfilter 4. Subsequently, a procedure by which a damping level (depth) andquality factor (Q) of the dip in the frequency to be removed isdetermined will be described.

First of all, an area of an area S1 defined by the curve Ca of the basicamplitude frequency characteristic and the curve Cd of the targetamplitude frequency characteristic in the frequency range from thefrequency f21 to the frequency f22 in FIG. 7 is detected. In FIG. 7, thearea S1 is illustrated as being hatched. Here, it is assumed that areaof the detected area S1 is T1.

Then, the magnitude of the amplitude level difference in the frequencyf2 between the target amplitude frequency characteristic (curve Cd) andthe basic amplitude frequency characteristic (curve Ca) is detected andis assumed as the damping level (depth) of the dip filter 4. Theamplitude level difference in the frequency f2 between the targetamplitude frequency characteristic (curve Cd) and the basic amplitudefrequency characteristic (curve Ca) is 2.5 dB, and the dip depth isassumed to be 2.5 dB.

Then, the quality factor (Q) of the dip is assumed to be 40. And, anarea of the dip is calculated from a shape of the dip (shape of the dipin the amplitude frequency characteristic view) obtained from theassumed dip depth and quality factor.

In FIG. 9, a curve Cg represents an amplitude frequency characteristicof the dip in which the center frequency is the frequency f2. In FIG. 9,an ordinate axis and an abscissa axis are logarithmic axes and indicatean amplitude level and a frequency, respectively. In FIG. 9, an area ofthe area S2 hatched by a number of parallel oblique lines is the area ofthe dip. Here it is assumed that the area of the dip calculated from theassumed depth and quality factor of the dip is T2. The area T1 iscompared to the area T2. If the area T2 is equal to or larger than thearea T1, then the assumed damping level and quality factor aredetermined as the damping level and quality factor of the dip in thefrequency to be removed in the dip filter 4.

If the area T2 is smaller than the area T1, the quality factor isdecreased by 0.1 and the area T2 is found. And, the area T1 is comparedto the area T2 again. If the area T2 is equal to or larger than the areaT2, then the assumed damping level and quality factor are determined asthe damping level and quality factor of the dip in the frequency to beremoved in the dip filter 4. Conversely, if the area T2 is smaller thanthe area T1, the quality factor is decreased by 0.1 and the area T2 isfound. Again, the area T1 is compared to the area T2. Thereafter, thequality factor is decreased by 0.1 until the area T2 becomes equal to orlarger than the area T1 in the same manner, and the damping level andquality factor with the area T2 being equal to or larger than the areaT1 are determined as the damping level and quality factor of the dip inthe frequency to be removed in the dip filter 4.

If the area T2 is still smaller than the area T1 even when the qualityfactor is decreased to a predetermined value (e.g., 1.5), the dampinglevel is thereafter increased by a predetermined value (e.g., 0.5 dB)without decreasing the quality factor. The damping level and qualityfactor with the area T2 being equal to or larger than the area T1 aredetermined as the damping level and quality factor of the dip in thefrequency to be removed in the dip filter 4.

Furthermore, if the area T2 is still smaller than the area T1 even whenthe damping level is increased up to a predetermined value (e.g., 12dB), the damping level and quality factor at that point are determinedas the damping level and quality factor of the dip in the frequency tobe removed in the dip filter 4.

In the manner as described above, based on the frequency f2 which is thefirst candidate frequency, the first frequency to be removed (centerfrequency of dip) which is to be set in the dip filter 4, and thedamping and quality factor of the frequency are determined.

Then, based on the frequency f3 which is the second candidate frequency,the second frequency to be removed (center frequency of dip) which is tobe set in the dip filter 4, and the damping and quality factor of thefrequency are determined by a similar procedure.

Since the frequency f1 has been already excluded from the candidatefrequencies, the frequencies to be removed (center frequency of dip)which are to be set in the dip filter 4 are the frequency f2 and thefrequency f3.

When there are a number of candidate frequencies, the frequencies to beremoved (e.g., 12 frequencies to be removed) which are capable of beingset in the dip filter 4 are determined by a similar procedure. If all ofthe frequencies to be removed (e.g., 12 frequencies to be removed) whichare capable of being set in the dip filter 4 are set in the dip filter4, the remaining candidate frequencies are not set in the dip filter 4as the frequencies to be removed.

In the manner as described above, the frequencies f2 and f3 to be set inthe dip filter 4 as the frequencies to be removed, and the dampinglevels (depth) and the quality factor (Q) of the dips in thosefrequencies are determined. By setting these characteristics as thecharacteristics of the dip filter 4 in the acoustic system of FIG. 1,resonance in the loud sound space 40 is prevented.

As described above, the area of the dip of the dip filter 4 issubstantially equal to the area formed by exceeding the basic amplitudefrequency characteristic from the target amplitude frequencycharacteristic, and in principle, the amplitude level difference in theresonant frequency (center frequency of dip) between the basic amplitudefrequency characteristic and the target amplitude frequencycharacteristic is set as the damping level of the dip of the dip filter4. By applying the characteristic of the dip filter 4 to the basicamplitude frequency characteristic, a characteristic which is extremelynear the target amplitude frequency characteristic is achieved.Therefore, the acoustic system of FIG. 1 including the dip filter 4having such a characteristic has an appropriate characteristic capableof preventing resonance without degrading a sound quality.

Thus far, with reference to FIGS. 1 through 9, the method of determiningthe frequency characteristic of the dip filter according to anembodiment of the present invention has been described.

Numerous modifications and alternative embodiments of the invention willbe apparent to those skilled in the art in view of the foregoingdescription. Accordingly, the description is to be construed asillustrative only, and is provided for the purpose of teaching thoseskilled in the art the best mode of carrying out the invention. Thedetails of the structure and/or function may be varied substantiallywithout departing from the spirit of the invention and all modificationswhich come within the scope of the appended claims are reserved.

INDUSTRIAL APPLICABILITY

In accordance with a method of determining a frequency characteristic ofa dip filter of the present invention, a characteristic of the dipfilter is appropriately determined without a need for experience orskills, and therefore are advantageous in technical fields of acousticequipment.

1. A method of determining one or both of a damping level and a qualityfactor of a dip filter adapted to remove a component of a resonantfrequency based on the one or both of the damping level and the qualityfactor, the method comprising: determining the resonant frequencydetected in a resonant space as a center frequency of a dip; finding abasic amplitude frequency characteristic based on a measurement valueobtained by outputting a loud sound wave of a predetermined measurementsignal from a speaker placed in the resonant space and by receiving theloud sound wave in a microphone placed in the resonant space; finding atarget amplitude frequency characteristic having a smoothness degree ona frequency axis which is larger than a smoothness degree of the basicamplitude frequency characteristic on the frequency axis, based on themeasurement value; identifying as a first frequency a frequency closestto the center frequency of the dip among frequencies at which a curve ofthe basic amplitude frequency characteristic and a curve of the targetamplitude frequency characteristic cross each other, the first frequencybeing lower than the center frequency of the dip; identifying as asecond frequency a frequency closest to the center frequency of the dipamong frequencies at which the curve of the basic amplitude frequencycharacteristic and the curve of the target amplitude frequencycharacteristic cross each other, the second frequency being higher thanthe center frequency of the dip; defining as a first area an area havingan outline defined by the curve of the basic amplitude frequencycharacteristic and the curve of the target amplitude frequencycharacteristic in a frequency between the first frequency and the secondfrequency, inclusive, when the curve of the basic and target amplitudefrequency characteristics are represented in an amplitude frequencycharacteristic view in which a logarithm axis indicating an amplitudelevel is an ordinate axis and an axis indicating a frequency is anabscissa axis; and defining as a second area an area of the dip formedwhen an characteristic of the dip is represented on the amplitudefrequency characteristic; determining the one or both of the dampinglevel and the quality factor of the dip based on a difference betweenthe basic amplitude frequency characteristic and the target amplitudefrequency characteristic in the center frequency and a frequency nearthe center frequency; and setting the dip filter to the determinedresonant frequency and the determined one or both of the damping leveland the quality factor so that the dip filter operates to remove thecomponent of the resonant frequency; wherein the center frequency of thedip is determined in such a manner that a resonant frequency with thehighest amplitude level of a second amplitude frequency characteristicis set as the center frequency of the dip among plural resonantfrequencies and remaining resonant frequencies are not set as the centerfrequency of the dip when the plural resonant frequencies detected inthe resonant space are included in the frequency range from the firstfrequency to the second frequency; wherein the second amplitudefrequency characteristic is obtained by outputting, from the speaker, aloud sound wave of a synthesized signal containing the measurementsignal and a signal output from the microphone and by receiving the loudsound wave in the microphone; and wherein the one or both of the dampinglevel or quality factor of the dip are determined so as to make thesecond area substantially equal to the first area.
 2. A method ofdetermining one or both of a damping level and a quality factor of a dipfilter adapted to remove a component of a resonant frequency based onthe one or both of the damping level and the quality factor, the methodcomprising: determining the resonant frequency detected in a resonantspace as a center frequency of a dip; finding a basic amplitudefrequency characteristic based on a measurement value obtained byoutputting a loud sound wave of a predetermined measurement signal froma speaker placed in the resonant space and by receiving the loud soundwave in a microphone placed in the resonant space; finding a targetamplitude frequency characteristic having a smoothness degree on afrequency axis which is larger than a smoothness degree of the basicamplitude frequency characteristic on the frequency axis, based on themeasurement value; determining as a first amplitude frequencycharacteristic an amplitude frequency characteristic which is obtainedby outputting, from a speaker placed in the resonant space, a sound waveof a predetermined measurement signal and by receiving the sound wave ina microphone placed in the resonant space; determining as a secondamplitude frequency characteristic an amplitude frequency characteristicwhich is obtained by outputting, from the speaker, a loud sound wave ofa synthesized signal containing the measurement signal and a signaloutput from the microphone and by receiving the loud sound wave in themicrophone; determining the one or both of the damping level and thequality factor of the dip based on a difference between the basicamplitude frequency characteristic and the target amplitude frequencycharacteristic in the center frequency and a frequency near the centerfrequency; and setting the dip filter to the determined resonantfrequency and the determined one or both of the damping level and thequality factor so that the dip filter operates to remove the componentof the resonant frequency; wherein the resonant frequency in theresonant space is detected based on comparison of the first amplitudefrequency characteristic and the second amplitude frequencycharacteristic.
 3. The method of claim 2, wherein a peak frequency atwhich an amplitude of the second amplitude frequency characteristic islarger than an amplitude of the first amplitude frequency characteristicis detected as the resonant frequency in the resonant space, from adifference between the first amplitude frequency characteristic and thesecond amplitude frequency characteristic.